1. Field of the Invention
The present invention relates to an electric power converter for converting an alternating electric power having a frequency into another alternating electric power having another frequency.
2. Description of the Prior Art
In FIG. 1, there is shown a conventional circulating current type cycloconverter for driving a three-phase electric motor 6 as an electric power converter. The cycloconverter comprises three positive converter devices 1 for passing a positive side of an output current, three negative converter devices 2 for passing a negative side of the output current, three pairs of three-phase transformers 3 whose primary coils are connected to an alternating electric power source (not shown) and whose secondary coils are connected to the positive and negative converter devices 1 and 2, three reactors 4 connected to the outputs of the positive and negative converter devices 1 and 2, for restraining circulating currents flowing in the positive and negative converter devices 1 and 2 and the transformers 3, and a phase-leading capacitor 5 connected to the primary coils of the transformers 3. Neutral or middle points of the reactors 4 are connected to three coils of the three-phase electric motor 6. Each positive converter device 1 comprises a pair of converters 7 and 8 connected to each other in series in two stages, and each negative converter device 2 comprises a pair of converters 9 and 10 connected to each other in series in two stages. A semiconductor element or a switching semiconductor element such as a gate turnoff element or a thyristor may be properly used for the converter. Further, the positive converter devices 1, the negative converter devices 2 and the transformers 3 are arranged and connected corresponding to three phases U, V and W.
A principle of an action of the cycloconverter described above will be described in detail.
Each positive converter device 1 and each negative converter device 2 output respective voltages in the same time, and an average voltage of these two voltages is applied through each reactor 4 to each coil of the motor 6. Symmetrical three-phase sinusoidal voltages having a certain frequency are output from the three pairs of converter devices 1 and 2 through the reactors 4, and the phases of the three-phase voltages are shifted 120 degree from one to another. Thus, three-phase sinusoidal currents are fed to the three coils of the motor 6 for driving the same. At this time, a voltage difference (the output voltage of the positive converters device 1 is equal to or larger than the output voltage of the negative converters device 2) arises between the output voltages of the positive and negative converter devices 1 and 2, and hence the circulating current flows in a closed circuit composed of the positive converter device 1, the reactor 4, the negative converter device 2 and the transformers 3.
In this cycloconverter, since a large reactive power is generated, in order to compensate this reactive power, the phase-leading capacitor 5 is connected to the primary coils of the transformers 3, thereby improving an input power factor. However, the reactive power generated in the cycloconverter varies depending on its operational conditions, and thus the input power factor cannot be maintained to be high in an entire operation range by a phase-leading capacitor having a certain value. Then, in order to solve this problem, the circulating currents or the reactive power which cannot contribute to the output of the motor, is so controlled to be a certain value, that the sum of the reactive power of the circulating currents and the reactive power generated in the cycloconverter may be a predetermined value. On this occasion, the capacity of the phase-leading capacitor 5 is selected so as to nullify the controlled whole reactive power, resulting in that it may be possible to operate the cycloconverter always at a high power factor such as the power factor in the entire operation range.
Next, a relation between the wire connections of the transformer 3 and the action of the cycloconverter will be described with reference to FIGS. 2 and 3.
As shown in FIG. 2, each of the transformers 3 includes one delta connection in one secondary coil and two star connections in the primary coil and the other secondary coil, and the phases of the voltages supplied to the converters 7 and 10 connected to the delta connection secondary coils are shifted to -30.degree. with reference to those of the star connection primary coils while the phases of the voltages supplied to the converters 8 and 9 connected to the star connection secondary coils are the same as those of the star connection primary coils. For instance, the phases of the voltages supplied to the converters 7, 8, 9 and 10 are -30.degree., 0.degree., 0.degree. and -30.degree., respectively.
In FIG. 3, there are shown three wave forms of the voltages at the outputs of the positive converter device 1 and the negative converter device 2 and at the neutral point of the reactor 4. The voltage having 30.degree. phase 12 pulses is output from the positive or negative converter device 1 or 2. At this time, the voltage to be supplied to the motor 6 or at the neutral point of the reactor 4 becomes the naught voltage on an average, as shown in FIG. 3. Hence, the motor 6 is not driven but is stopped, and only the circulating currents corresponding to the phase-leading capacitor 5 circulates in the circuits.
In the above-described conventional cycloconverter, since there is no current to be supplied to the motor upon stopping the motor, the reactive power corresponding to the phase-leading capacitor is all generated by only flowing the circulating currents. In this case, large currents flow only in the closed circuits each composed of the positive converter device 1, the reactor 4, the negative converter device 2 and the transformers 3. A large quantity of higher harmonics of 6n.+-.1 orders, wherein n is an integer, are generated.
In general, in the main circuit connection shown in FIG. 1, since a winding phase of one secondary coil of the transformer 3 is shifted to 30.degree., the higher harmonics of 12n.+-.1 orders, in which n is an integer, are generated. Further, it is generally known that the amount of the current of the generated higher harmonic is 1/(6n.+-.1) time, in which n is an integer, as much as that of a fundamental wave. This current of the higher harmonic brings about a distortion or deformation of a voltage wave form of the electric power source, resulting in giving bad influences to electric equipments, apparatus or machinery connected to the same electric power source. For example, the current of the higher harmonics of the eleventh and thirteenth orders cause an overcurrent of a rated current of a resistor in an electric power source system, with the result of generating an overheat.